Changes in streamflow regime due to anthropogenic regulations in the humid tropical Western Ghats, Kerala State, India

Regulation of streamflow by a reservoir creates a flow regime much different from the pre-impoundment period flow regime. Hydro-Electric Projects (HEPs) commissioned in the Western Ghat regions of the Kerala State, India during the last four decades caused considerable changes in the flow regimes of the rivers of the Kerala State in southwest India. In this paper, the Indicators of Hydrologic Alteration (IHA) approach proposed by Richter et al. (1996) is used to analyze flow regime changes in the Periyar and Muvattupuzha Rivers, due to the construction of the Idukki (1976), Idamalayar (1987) and Lower Periyar (1997) HEPs in the high ranges of the Western Ghats. Normal rainfall years (annual rainfall values within mean +/- 0.75 standard deviation limits) are only considered in the analysis to focus on hydrologic alterations due to human activities. The mean hydrologic alteration in the Periyar River (deviation from the pre-development hydrologic indicator values) after commissioning of three HEPs is 35%. Inter-basin water transfer after power generation from the Idukki HEP resulted in a higher discharge in the adjacent Muvattupuzha River, leading to considerable changes in the hydro-environment (mean hydrologic alterations varying between 57 to 63%). IHA parameters showing hydrologic alterations above the 67(th) Percentile were further analyzed. For each of the pre-construction hydrologic parameters +/- 1 standard deviation from the mean is set as the upper and lower management target limits. The values of each IHA parameter beyond these targets are considered as non-attainment. Considerable hydrologic alterations are observed, especially for low flows in both basins. Inter-basin transfer induced larger changes in flow parameters compared to intra-basin regulations. The study shows that under a proper water release and diversion scheme, the non-attainment of IHA parameters (values fall beyond the target limits) can be reduced. The findings of the study will be greatly beneficial to regional water management and restoration of an eco-environmental system in the humid tropical region.